1. Field of the Invention
The present invention relates to a self-propelled vehicle having right and left drive wheels which are driven and controlled independently of each other.
2. Description of the Related Art
According to an example of such self-propelled vehicle as above, the vehicle includes a maneuvering unit which is operated by a driver, a drive unit having a first drive section and a second drive section which are driven and controlled independently of each other by a drive wheel control command, a drive wheel unit having a left drive wheel driven for traveling by the first drive section and a right drive wheel driven for traveling by the second drive section, at least one caster wheel which is controlled for steering angle by a caster wheel control command, a drive wheel control section for generating the drive wheel control command based on an operational amount from the maneuvering unit and a caster wheel control section for generating the caster wheel control command for controlling the steering angle of the caster wheel.
With the above-described self-propelled vehicle, by increasing the difference between speeds (circumferential speeds) of the left drive wheel and the right drive wheel, a small turn is made possible. Further, by rotating the left drive wheel and the right driven wheel in different directions from each other, a “super-skid turn (spin turn)” or so-called “zero turn” is made possible. Hence, the vehicle has good small turn performance. For this reason, the vehicle can be used advantageously as a fork lift, a grass mower, etc. However, when the traveling surface is a sloping surface, an inclined downward force is applied to the wheels, so the vehicle tends to make a turn inclinedly downward. Especially, when the vehicle is traveling by inertia, no torque is not transmitted to the wheels, so the vehicle will make an inclinedly downward turn inadvertently.
As a solution to the above-described problem, according to a riding type grass mower vehicle disclosed in the Japanese Unexamined Patent Application Publication No. 2008-168871 (JP 2008-168871 A) (paragraphs [0012-0033, 0080-0106], FIG. 3, FIG. 4), the vehicle includes at least two main drive wheels and a caster wheel, the main drive wheels being driven by a traveling power source. The vehicle further includes a switching means for switching to either a forcible steering mode in which the caster wheel is forcibly steered by a steering power source, or to a free steering mode in which the caster wheel is rendered freely steerable by blocking the power transmission from the steering power source. With this vehicle, in the case of traveling on or traversing a ground surface or grass field having a slope (bank) angle (such traveling as this will be referred to as “a bank traversing travel” herein), the mode is switched over to the forcible steering mode, whereby it becomes possible to prevent the caster wheel to be oriented more downward than the direction desired by the driver. For instance, an optimal steering angle optimal for the turn determined by an operational lever is forcibly applied to the caster wheel. Further, JP 2008-168871 A discloses that there is provided a switching means for switching from a first drive mode in which only the main drive wheels are driven and a second drive mode in which both the main drive wheels and the caster wheel are driven, so that during a hill climbing travel of the vehicle on a sloping surface, if there occurs a slip on the grass surface in the main wheels by an amount exceeding a predetermined value, both the main drive wheels and the caster wheel are driven. There is also disclosed that in case the grass mower vehicle is stopped on a sloping surface, an electric motor is controlled so as to generate a torque in the vicinity of zero rotational speed of the motor, in accordance with a sloping angle detected by a slope sensor.
With the above-described self-propelled vehicle according to JP 2008-168871 A, the vehicle is effective for restricting the inclinedly downward turning tendency which occurs during an inertia travel on a sloping surface. However, no consideration is given to such use condition in which traveling on a sloping surface is effected with driving the drive wheels and with a similar maneuvering feel for the driver to that for traveling on a flat terrain. When a self-propelled vehicle having a free caster wheel and right and left drive wheels effects a bank traversing travel, only providing the caster wheel with an optimal steering angle for the turn determined by the operational lever is not sufficient. It is additionally needed to take into consideration the gravitational load applied by the bank of the vehicle; that is, it is necessary to provide the caster wheel with the optimal steering angle based on a detected vehicle bank degree.
As another solution to the above-described problem, in the Japanese Unexamined Patent Application Publication No. 2009-255840 (JP 2009-255840 A) (paragraphs [0012-0026, 0098-0109], FIG. 12-FIG. 13), there is proposed an electrically powered ground work vehicle including right and left drive wheels as main drive wheels that are driven for traveling independently by right and left electric motors, at least one caster wheel as a freely steerable steering wheel, a utility work implement driven for effecting a ground work, an acceleration operational member for providing instruction for acceleration, a turn operational member for providing instruction for turn, and a control section for braking the right and left wheels for regeneration by controlling a regeneration braking drive section for regenerating electric power from the right and left electric motors to an electric power source unit, when the acceleration operational member is not operated during vehicle travel. The control section of this electrically powered ground work vehicle controls braking forces of the right and left wheels in such a manner that the braking force for the one wheel which is positioned on the inner side of the turn is greater than the braking force for the other wheel which is positioned on the outer side of the turn, when the acceleration operational member is not operated and a turn command is inputted from the turn operational member.
Specifically, there are provided a roll angle detecting means (a roll angle sensor) for detecting a state wherein the vehicle is pivotally inclined relative to an axis extending through the center of gravity of the vehicle oriented along the fore/aft direction, and a roll angle correcting means. With the roll angle correcting means, during an inertia travel or a pedal braked condition when the accelerator pedal is not operated during travel and when the roll angle θ indicated by the signal of by the roll angle sensor is not 0 (zero), the correcting means corrects the braking forces for the respective right and left wheels according to this roll angleθ so that the wheels may be oriented to the direction corresponding to the operational direction of the steering operational member at the time of flat terrain travel when the roll angleθ is 0 (zero). With this arrangement, the right and left wheels 1 are driven for traveling independently of each other. For instance, in the electrically powered ground work vehicle capable of turning due to a rotational speed difference between right and left electric motors, stable traveling of the vehicle can be ensured even when the acceleration operational member is not operated during traveling such as in an inertial travel.
With the above self-propelled vehicle according to JP 2009-255840 A, this vehicle is effective for restricting the inclinedly downward turning tendency of the vehicle which occurs during an inertial travel on a sloping surface. However, no consideration is given to such use condition similar to the maneuvering on a flat terrain, when the wheels are driven on a sloping surface. In the case of traveling on or traversing a ground surface or grass field having a bank angle (such traveling will be referred to as “bank traversing travel” herein), it is necessary to control the driving of the right and left drive wheels with taking the inclinedly downward turning tendency into consideration. Such maneuvering requires experience and skill.
Further, according to a self-propelled vehicle known from the Japanese Unexamined Patent Application Publication No. 2008-168869 (JP 2008-168869 A), whenever a desired travel speed and turning condition are commanded by a maneuvering unit, the vehicle obtains a target speed and a driving torque required for obtaining such target speed for each one of a pair of electric motors so that the vehicle may travel at the commanded travel speed and turning condition and the respective electric motors are controlled so as to provide the required target speeds and also to output the required torques.
With this self-propelled vehicle according to 2008-168869, as the desired travel speed and turning condition are commanded by the maneuvering unit and the target speeds for the right and left drive wheels are set separately, not only straight forward travel, reverse travel, but also turning travels in the forward and reverse directions respectively are made possible. So, the vehicle may be employed as a grass mower or the like advantageously. However, in order that the vehicle may always travel at a commanded travel speed and turning condition, a target speed for each one of the pair of electric motors and the drive torque for obtaining this target speed are obtained and the operations of the respective electric motors are controlled so as to provide the required target speeds and the required drive torques. For this reason, though will be explained in greater details later, if a control command speed and a control command drive torque are issued based on information from the maneuvering unit in an output restricted range which exceeds the rated range (e.g. a short period operation possible range or an output impossible range described above), the turning intended by the operator may not be possible.
In view of the above-described state of the art, there is a need for a self-propelled vehicle capable of being freely driven for traveling and traversing a sloping surface without requiring experience or skill. Further, it is desired that the turning travel intended by the driver may be readily effected based on information from the maneuvering unit even when a control command speed and a control command drive torque are issued in an output restricted range which exceeds the rated range.